In the simplest case the bus interface unit is a circuit board with corresponding inputs and outputs for connection to the two-wire current loop and to a data bus. The modem unit of the bus interface unit can for example be a CAN bus interface with a CAN bus transceiver, an I2C interface or quite generally a serial or parallel interface. The modem unit can for example be a compact integrated electronic component.
The transducer and the position controller are typically employed in plant and automation technology, especially in the chemical industry, petrochemicals and mining. The analog input module as well as the analog output module are especially provided for plugging into a control center computer or process computer. The latter can for example be Personal Computers (PC). Alternatively the analog input and output modules can be components of a Programmable Logic Controller (PLC). They can alternatively be embodied as decentralized input and/or output components for fitting to a top-hat rail.
Transducers, such as type SITRANS P from Siemens, and also position controllers, such as type SIPART PS2 from Siemens, are operated for transmission of a measured value or a setpoint value to a two-wire current loop. Such a transducer and position controller is also supplied with electrical energy for feeding the electronic switching circuits via the same two-wire current loop. The feeding distant end can be an analog input module or an analog output module of a control center computer, a process computer or a PLC.
A position controller is usually a device for positioning valves in automation technology. It is primarily used in the process industry in order to check the production sequences. The position controller can have a linear drive or a rotation drive, to enable it to actuate linked system components, such as pushers, brake elements, air valves and such like. In general a position controller converts an electrical input signal into a corresponding output signal. Furthermore all position controller types have position feedback which serves to determine the actual position.
The range of measured values of a measured value to be transmitted, such as a temperature, a force, a pressure value or similar, is usually transferred in an assigned current value range of the two-wire current loop of 4 mA to 20 mA (4/20 mA Standard). The impressed current involved is a direct current. The range of measured values to be transmitted is mapped in such cases into a current value range of 16 mA, so that a permanent power supply of the signal circuit of the transducer and of a sensor or an actuator connected to it, such as of a valve, of the position controller is possible.
The direct current or normal signal transmitted over the two-wire current loop is relatively insensitive to electromagnetic disturbances. The maximum length of the two-wire current loop, with corresponding compensation of the line losses, meaning with corresponding raising of feed voltage, can amount to 1000 m and more. At least one measurement resistance in the range 50 to 250 Ohm is connected in the two-wire current loop. This is needed for the transducer in order to impress a current value corresponding to the measured value into the two-wire current loop by means of a closed control loop. With the position controller this measurement resistance is needed to enable the setpoint value transmitted by the analog output module to be read out. The same also applies for an analog input module or analog output module.
The analog field bus standardized on the basis of the 4/20 mA two-wire current loop can be expanded using a so-called HART® (Highway Addressable Remote Transducer) communication. HART® is a standardized, widely-used communication system for setting up industrial digital field busses. It allows digital communication between a number of user (field devices) over a common digital field bus via the two-wire current loop according to the older 4/20 mA standard. Existing lines according to the 4/20 mA standard can be used directly and both systems can be operated in parallel.
In accordance with the HART® protocol an alternating current signal with a current amplitude in the range of 0.8 mA to 1 mA is modulated onto the standard signal, i.e. the direct current signal of the two-wire loop current. The modulation is undertaken by means of an FSK (Frequency-Shift-Keying) method.
In such cases, for the binary 0/1 sequence of a data word to be transmitted, a modulation frequency of 2200 Hz for the value “0” and 1200 Hz for the value “1” is used according to its binary values. To guarantee a reliable reception, in accordance with the HART®-protocol an overall resistance of the two-wire current loop inclusive of the line resistance in the range between 230 Ohm and 1100 Ohm is specified.
Because of the spatial distribution and different feeding of the field devices, such as the transducers and position controllers for example, a potential separation between the two-wire current loop and the data bus side of the bus interface unit is frequently necessary. This avoids possible negative influences on the data transmission, such as inductive coupling-in earth loops or EMC couplings into the current loop.
Usually a transformer is used for potential separation. From the technical standpoint this is a transformer with one or more primary-side or secondary-side windings. In such cases the loop current flows permanently with its predominant direct current component in the range of 4 mA to 16 mA through the primary winding and magnetizes the core of the transformer. The transformer core is thus to be designed for the maximum loop current in respect of signaling transmission behavior. The exterior dimensions of the transformer are correspondingly large by comparison with the integrated modem unit.